[...] Mayr's (2004) hypothesis that
penguins and plotopterids might actually be closely related (sister taxa).
Gerald Mayr: Tertiary plotopterids (Aves, Plotopteridae) and a novel
hypothesis on the phylogenetic relationships of penguins (Spheniscidae),
Journal of Zoological Systematics and Evolutionary Research 43(1), 61 -- 71
(2005)
Abstract:
"Plotopterids (Aves: Plotopteridae) are extinct wing-propelled diving birds
which exhibit a strikingly similar wing morphology to penguins
(Spheniscidae), but also share derived characters with 'pelecaniform' birds
that are absent in penguins. The similarities between Plotopteridae and
Spheniscidae have hitherto been attributed to convergence, and plotopterids
were considered to be most closely related to the 'pelecaniform'
Phalacrocoracidae (cormorants) and Anhingidae (anhingas). However, here I
show that assignment of plotopterids to 'pelecaniform' birds does not
necessarily preclude them from being the sister taxon of penguins. Cladistic
analysis of 68 morphological characters resulted in sister group
relationship between Plotopteridae and Spheniscidae, and the clade
(Plotopteridae + Spheniscidae) was shown to be the sister taxon of the
Suloidea, i.e. a clade including Sulidae (boobies and gannets),
Phalacrocoracidae, and Anhingidae. Derived characters are discussed which
support this novel hypothesis. Paedomorphosis probably accounts for the
absence of derived characters in penguins that are shared by Plotopteridae
and members of the Steganopodes. Plotopterids exemplify the importance of
fossil birds for analyzing the phylogenetic relationships of modern taxa
that exhibit a highly apomorphic morphology."
This is the first publication to compare plotopterids to Paleogene, as
opposed to extant, penguins and to compare more than the wing skeleton. It
also contains the first cladogram that includes Plotopteridae (17 ingroup
taxa, 68 characters).